Many products, particularly food products are sensitive to the presence of oxygen and/or the loss or absorption of water. These products are susceptible to deterioration, when packaged, due to oxygen and/or moisture absorption or loss through the wall of the package. Attempts to solve the problem have led to the widespread use of oxygen barriers and/or moisture barriers in packaging materials. Typical moisture barriers include polyethylene and polypropylene. Suitable oxygen barriers include EVOH, PVOH, Nylon and blends thereof. Vinylidene chloride--vinyl chloride copolymers and vinylidene chloride--methyl acrylate copolymers are suitable as both moisture and oxygen barriers.
A problem with conventional barrier materials is that due to their high cost or their unstable structural characteristics or other weaknesses, it is difficult to fabricate commercial packaging solely out of barrier materials. For instance, EVOH, while having superior oxygen barrier properties, suffers moisture problems because of the many hydroxyl groups in the polymer. Other barrier materials are so expensive that to manufacture structures solely from those barriers would be cost prohibitive. Accordingly, it has become a common practice to use multilayer structures, whereby, the amount of expensive or sensitive barrier material may be reduced to a thin layer and an inexpensive polymer can be used on one or both sides of the barrier layer as structural layers. In addition, the use of multilayer structures permits the barrier layer to be protected from deterioration by structural layers on one or both sides of the barrier layer.
Although multilayer structures containing a barrier layer may be cheaper and stronger than a single layer of barrier materials, such structures are more complicated to manufacture than single-layered ones. In addition, multilayer structures comprised of layers of a variety of different materials may be opposed in some instances on environmental grounds, they may be more difficult to recycle since it is often difficult and expensive to separate the layers. In addition, reducing the thickness of the barrier layer in a multilayer structure can reduce the barrier properties of the film. Accordingly, there is a need for a single-layer packaging material with suitable barrier properties but without the cost or structural weaknesses of packaging made solely from a barrier material. There is also a need for additional multilayer structures having improved barrier properties wherein, the barrier material is reduced to a thinner layer and replaced in part by inexpensive structural layers. These structures have the same barrier properties of prior art barriers but at lower cost due to a decrease in the amount of expensive barrier material used.
In addition to barrier properties, it is frequently desirable to use materials which have oxygen absorption capabilities. These oxygen absorption or oxygen scavenging materials are useful in reducing the amount of oxygen that contaminate the product packaged in the container. An example of oxygen scavenging materials and methods of using them is disclosed in U.S. Pat. No. 4,425,410 to Farrell et al, the disclosure of which is hereby incorporated by reference herein. Another useful aspect of oxygen absorbing material is that such materials can reduce residual oxygen which is trapped in the headspace of a container during sealing, thereby preventing it from having a deleterious effect on the packaged products.
A material that is commonly used in packaging applications is polyethylene terephthalate resin, hereinafter referred to as PET. While PET has a number of valuable properties in packaging, it does not have as good a gas barrier property as is frequently required or desired in many applications. For example, although PET has good carbon dioxide barrier properties for soft drinks, it has not been found useful in packaging such products as beer because beer rapidly loses its flavor due to oxygen migration into the bottle. Similar problems are encountered with citrus products, tomato based products and aseptically packed meat. A packaging material with physical properties similar to PET is polyethylene naphthalate (PEN) which is 3-20 times more effective as a barrier but is considerably more expensive.
In order to enhance polyester's gas barrier properties, polyesters have been used in a multilayer structure in combination with a layer having excellent gas barrier properties such as EVOH. However, multilayer structures employing polyester, such as PET, frequently have adhesion problems between the polyester and the barrier layer which frequently leads to delamination over time.
One approach to enhancing the gas barrier property of PET is to use a resin mixture which includes PET and a xylylene group containing polyamide resin. Such resin materials are disclosed in U.S. Pat. No. 4,501,781 to Kushida et al. One of the considerations encountered with such blends accordingly to Kushida is that there is a limit to the amount of xylylene group-containing polyamide resin that may be present in the PET blend. Kushida indicates that amounts of xylylene group-containing polyamide resin greater than 30% by weight causes the container to become a laminated foil structure which is susceptible to exfoliation between the foil layers of the container.
According to Kushida, the permeation of oxygen gas through the walls of a container is less when the container is made wish PET and a xylylene group-containing polyamide than when the container is made solely of PET. Kushida reports that a bottle shaped container made with PET-xylylene group-containing polyamide measured 0.0001 cc of oxygen permeation per day compared to 0.0180 cc of oxygen permeation per day for a container made with PET.
A preferred xylylene group-containing polyamide resin in the present invention is an aromatic polyamide formed by polymerizing meta-xylylene-diamine (H.sub.2 NCH.sub.2 --m--C.sub.6 H.sub.4 --CH.sub.3 NH.sub.2) with adipic acid (HO.sub.2 C(CH.sub.2).sub.4 CO.sub.2 H). The most preferred such polymer is manufactured and sold by Mitsubishi Gas Chemicals, Japan, under the designation MXD6 or MXD6 nylon.
In U.S. application Ser. No. 07/472,400 to Hong et al., the gas barrier property of polyester is enhanced by blending polyester with xylylene group-containing polyamide and a transition metal catalyst. Preferred embodiments include blends of PET/MXD6/Cobalt and exhibit superior oxygen barrier and oxygen absorption characteristics that were not present in the prior art structures. However, the structures in this invention are not as clear as the prior art structures. Hong discloses that it is believed that the high orientation of the blend increases the surface areas and interface between PET and MXD6 nylon so that there are a greater number of sites at which a reaction or an absorption of oxygen catalyzed by the transition metal catalyst takes place. This increased surface area and interface between PET and MXD6 nylon also causes a change in the refractive characteristics of the materials and results in an increased diffusion of light passing through the structures. The disclosures made in the Hong application are hereby incorporated by reference herein.
In U.S. Pat. No. 4,407,873 to Christensen et al., the need for the proper selection of materials in films used in retort applications is discussed. Common to the requirements of retort pouch packaging is the requirement that the filled and sealed package be subjected to sterilizing conditions of relatively high temperature after the pouch is filled with product and sealed. Typical sterilizing conditions range in severity up to about 275.degree. F. with residence times at that temperature of as much as 30 minutes or more. Such conditions impose severe stresses on the packages. Many packaging structures provide excellent protection for the package contents at less severe conditions. For example, relatively simple packaging structures for packaging requiring the ability to withstand boiling water, such as at 212.degree. F. are readily available from several suppliers. When sterilizing conditions are required, however, most of these packages fail to survive the processing. Typically, problems are encountered with excessive weakening or failure of the heat seals about the periphery of the pouch. Also certain weaknesses or separations may develop between the layers in the multiple layer sheet structure. In addition, the high humidity experienced during the sterilizing process can change the chemical or structural properties of some materials.
While Hong reports improved barrier properties using PET/MXD6/cobalt blends, there is a further need for oxygen barriers of greater clarity. In addition, there is a need for compositions which can he used in retort application in addition to acting as a clear oxygen barrier material. Thus, it is an object of the present invention to provide an improved monolayer barrier structure that satisfies both clarity and retort functions.
It is also an object of the present invention to provide a clear monolayer barrier structure that has barrier properties superior to known barrier materials.
It is a further object of the present invention to provide a multilayer structure having a layer comprising an MXD6/cobalt blend that does not delaminate under conditions of high relative humidity.